1. Field of the Invention
The present invention relates to a binariation device such as an auto slice circuit that recovers original binary data consisting of digital signals from reproduction signals (high-frequency signals) obtained by reading digital signals from a recording medium such as a magnetic recording medium, an optical recording medium, or an magneto-optical recording medium.
2. Description of the Related Art
In reproduction devices for optical recording media such as compact disks (CDs) and digital video disks (DVDs), binarization devices have conventionally been employed to recover original data from reproduction signals of the recording media.
FIG. 34 shows the structure of an auto slice circuit that is an example of a conventional binarization device.
In this auto slice circuit, a binarization unit 10 binarizes an input signal. Based on the binary signal from the binarization unit 10, a slice level setting unit 11 sets a slice level signal, and returns the slice level signal to the binarization unit 10. The binarization unit 10 then compares the sizes of the returned slice level signal and the input signal to carry out a binarization process.
In the following, the conventional auto slice circuit will be described in greater detail.
FIG. 35 is a circuit diagram showing an example of the internal structure of the slice level setting unit 11 of the auto slice circuit shown in FIG. 34.
The slice level setting unit 11 includes an integrator 12 and a subtractor 13. An input signal is input into the binarization unit 10, and the output of the binarization unit 10 is input into the integrator 12. The subtractor 13 carries out a subtraction process between a DC offset signal and the output signal of the integrator 12. The result of the subtraction process is returned as the slice level signal to the binarization unit 10.
Next, the operation of the conventional auto slice circuit shown in FIG. 35 will be described in greater detail.
FIGS. 36A through 36E are waveform charts showing the waveforms of input signals and output signals of each part of the auto slice circuit of FIG. 35. FIG. 36A is a waveform chart of the input signal to be input into the binarization unit 10. FIG. 36B is a waveform chart of the binary signal to be output from the binarization unit 10. FIG. 36C is a waveform chart of the integral signal to be output from the integrator 12. FIG. 36D is a waveform chart of the slice level signal to be output from the subtractor 13. FIG. 36E is a waveform chart of the input signal and the slice level signal to be input into the binarization unit 10.
An input signal of the waveform shown in FIG. 36A is input into the binarization unit 10. Assuming that the slice level signal of the binarization unit 10 is initially 0(V), the binarization unit 10 compares the input signal with the slice level signal of 0(V). If the input signal is greater than 0(V), the binarization unit 10 outputs such a binary signal that the output voltage becomes 0(V). If the input signal is smaller than 0(V), the binarization unit 10 outputs such a binary signal that the output voltage becomes VR (+5(V), for example), as shown in FIG. 36B.
From a recording medium, a signal to lower the voltage of a recording area (or a mark part) relative to the voltage of a non-recording area (or a space part) is obtained. Assuming that a signal obtained by reversing the polarity of the above signal is used as an input signal for the auto slice circuit, the determination on the input signal shown in FIG. 36A based on the slice level signal (the slice level is assumed to be 0(V)) shows that the space length is smaller than the mark length.
The integrator 12 averages the binary signal of the waveform shown in FIG. 36B to obtain the integral signal of voltage V1 shown in FIG. 36C. The subtractor 13 then carries out a subtraction process between the integral signal of voltage V1 and a DC offset signal obtained by halving the high-level voltage VR (assumed to be 5(V), for example) of the binary signal, to obtain such a slice level signal as to make V0 equal to “VR/2−V1”. Here, the DC offset signal can be generally expressed as “(low-level voltage)+[(high-level voltage)−(low-level voltage)]/2”.
The slice level signal is returned to the binarization unit 10, which then compares the input signal with the voltage V0. If the input signal is greater than V0, the binarization unit 10 outputs a binary signal of 0(V). If the input signal is smaller than V0, the binarization unit 10 outputs a binary signal of +5(V).
By this returning operation, the slice level signal is set so that the space length becomes equal to the mark length with respect to the input signal as shown in FIG. 36E, and the original data are then determined from the reproduction signal of the recording medium, i.e., the reproduction signal is binarized.
When information is recorded on an optical recording medium such as an optical dick, an operation called “calibration” is normally carried out. The “calibration” includes the steps of test writing prior to the actual signal recording, checking the signal quality of the test writing, and thereby obtaining an optimum strength for the laser beam for recording. An optical recording medium such as a CD or a DVD has an area for the test writing on the innermost periphery of the disk. This area is called a “power calibration area (PCA)”, and the series of pre-recording operations is called an “optimum power control (OPC)” operation.
An example of the OPC is carried out as follows.
First, a test signal is recorded in the PCA, with the laser beam strength being varied stepwise or continuously. The recorded part is then reproduced, and the location of the optimally recorded part is determined from the HF signal quality. The strength of the laser beam with which the test signal has been recorded at the optimally recorded location is then set as the optimum laser beam strength for recording. The quality check of the reproduction HF signal is carried out by detecting the asymmetry of the HF signal.
FIG. 37 is a block diagram showing the structure of a conventional circuit that detects the asymmetry of a reproduction signal obtained by reproducing a test signal recorded with varied recording power, and determines the optimum recording power from the asymmetry. FIG. 38 is a waveform chart of the output signal of a high-pass filter 20 of the circuit shown in FIG. 37.
A test signal is normally recorded on an optical disk, with the recording power being gradually varied. The area in which the test signal is recorded is irradiated with a reproduction beam, and the reflection beam is detected. As shown in FIG. 37, the high-pass filter 20 cuts off the direct current component of the HF signal obtained as a light reception signal from the detected reflection light. As a result, a signal of the waveform shown in FIG. 38 is obtained.
A top peak detecting circuit 21 and a bottom peak detecting circuit 22 respectively perform an analog operation to detect the top peak (the peak on the plus(+) side) At and the bottom peak (the peak on the minus(−) side) Ab of the HF signal of each recording power.
An asymmetry calculation circuit 23 calculates the asymmetry β of the HF signal of each recording power from the following formula: β=(At+Ab)/(At−Ab).
A determination circuit 24 selects the recording power with which the closest asymmetry to the optimum asymmetry among the asymmetries β is obtained, and determines the selected strength for recording power to be the optimum recording power. By carrying out the actual recording with this optimum recording power, an excellent reproduction signal quality can be obtained.
The optimum recording power for the recording medium is determined by carrying out the OPC in the above described manner. However, optimum recording is not always guaranteed by this conventional manner for the following reasons. First, the variation pitch of the recording power set for the test recording is generally in the range of 0.5 to 1.0 mw, which is too large. Second, there are changes in the recording power and wavelength in an actual recording operation. Third, the recording sensitivity and characteristics vary with recording locations. Finally, if not properly chosen, some recording methods might adversely affect the recording result. This means that carrying out the OPC does not realize optimum recording, and only preferable recording conditions that can be selected within the recording device are selected.
More specifically, there is a problem with a recording medium having mark lengths recorded thereon. With such a recording medium, the jitter and the error rate dramatically changes with changes in recording power. Even if the recording power only slightly differs from the optimum recording power, the jitter and the error rate cannot satisfy the required standards. This problem is referred to as a “narrow recording power margin”.
To solve the above problem, it is ideal to produce a circuit having a minutely designed structure. However, since there are numerous parameters to be controlled and those parameters strongly affect one another, it is very difficult for a recording device to automatically set the parameters for optimum recording.
Furthermore, a recording medium is not necessarily reproduced by the recording and reproducing device that carried out the recording on the recording medium. Even if optimum recording is carried out on the recording medium by the recording and reproducing device, there is no guarantee that the recorded data can be reproduced in an optimum condition by a reproducing device, because the characteristics of the pick-up head (PUH) of the reproducing device are not identical to the characteristics of the PUH of the recording and reproducing device that carried out the recording. Here, the characteristics of a PUH include the wavelength, the numerical aperture (NA) of the lens, the rim intensity, and the beam diameter.
Although the jitter is important as a part of the reproduction characteristics, the error rate should be considered to be more important. It is wrong to consider that the dependencies of the jitter and the error rate on the recording power are identical, and that the error rate becomes lowest with the recording power with which the jitter becomes smallest, because the amplitude centers of all the recording mark lengths do not match with one another with a certain recording power.
Although to restrain the jitter at the smallest level has been considered to be the most important factor for optimum recording, a recording condition in which the jitter becomes smallest does not guarantee optimum recording. Since the jitter is a value representing a shift from the center value in standard deviation, the jitter exhibits a very small value if there exists only a small amount of data of large shifts from the center value among a large amount of data of small shifts from the center value. For this reason, even a very small jitter is not necessarily desirable, unless the error rate is at its lowest or is very low.
In other words, as long as the error rate is very low, the jitter may have a great value. However, a jitter of a great value might lead to changes in characteristics of the recording and reproducing device and its environment, or to an increase in the error rate due to a long storage period. Therefore, it is necessary to maintain the jitter within a certain range.